The techniques of thin-film deposition of semi-conducting materials such as hydrogenated amorphous silicon (aSiH), on glass insulating supports for example, make it possible to construct matrices of photosensitive points that can produce an image on the basis of visible or near-visible radiation. These matrices can nonetheless be used within the framework of the detection of radiological images. For this purpose, it suffices to interpose a scintillator screen between the X-ray radiation and the matrix so as to convert the X-ray radiation into luminous radiation in the band of wavelengths to which the photosensitive points are sensitive.
The photosensitive points which form these matrices generally comprise a photosensitive element associated with an element fulfilling an on/off switch function. The photosensitive point is mounted between a row conductor and a column conductor. Depending on requirements, the photosensitive device then comprises a plurality of photosensitive points laid out as a matrix or a strip.
The photosensitive element commonly consists of a diode, mounted in series with the on/off switch element. The on/off switch element may be for example a so-called switching diode whose “closed” or “passing” state corresponds to the bias which turns it on in forward mode, and whose “open” or “blocked” state corresponds to its reverse bias. The two diodes are mounted with opposite directions of conduction, in a so-called “head-to-tail” configuration. Such an arrangement is well known, notably from French patent application 86 14058 (publication No. 2 605 166) wherein are described a matrix of photosensitive points of the type with two diodes in “head-to-tail” configuration, a method for reading the photosensitive points and a way of constructing a photosensitive device such as this. The amorphous semi-conducting material shows some memory effect. This is related to its amorphous structure which comprises a large number of traps, many more than in crystalline materials. These traps are structural defects which extend over the whole of the forbidden band. They retain charges engendered during a useful image capture, in particular during exposure of the photosensitive points to luminous radiation. The material memorizes an image corresponding to a given luminous radiation and restores charges relating to this image in the course of the reading of the following image or indeed of several following images. The quality of the images enjoys the effects thereof.
Moreover, the semi-conductor components used in a matrix or a strip of photosensitive points are not all identical and the photosensitive device into which this matrix or this strip is integrated then inherently possesses inhomogeneities which result in impaired zones which vary over time.
To try to obtain a useful image of optimal quality, a correction of the useful image is performed on the basis of a so-called offset image (known in the corresponding French terminology as a black image), captured for example at the start of an operating cycle or subsequent to a useful image capture. This offset image is the image obtained while the photosensitive device is exposed to a signal of zero intensity and corresponds to a sort of background image. The offset image varies as a function of the electrical state of the components of the photosensitive points and of the dispersion of their electrical characteristics. The useful image is that read while the photosensitive device has been exposed to a useful signal which corresponds for example to an exposure of the scintillator to X-ray radiation. It encompasses the offset image. The correction of the useful image then consists in performing a subtraction of the offset image from the useful image. This correction is reliable only if the offset image has not varied between the moment at which it was captured and the moment at which the useful image is captured. This implies that the photosensitive points are in the same electrical state just before the offset image is captured and just before the useful image is captured.
However, the photosensitive points are generally sensitive to electromagnetic disturbances. In certain cases of use of the photosensitive device, electromagnetic disturbances are inevitable. This is the case for example when the photosensitive device is used simultaneously with an electric bistoury within the framework of interventional radiography. Consequently, the electrical state of the photosensitive points may vary between useful image capture and offset image capture. If the disturbance is permanent and periodic, strips can appear on the image formed by the photosensitive device, unless the frequency of the electromagnetic disturbance is very slow compared with the image capture frequency.
To decrease the impact of electromagnetic disturbances on the images formed by a photosensitive device, it is possible to render the photosensitive device insensitive to these disturbances, notably by designing a shielding and by removing the current loops. However, this is not always possible in the presence of heavy constraints, for example mechanical (weight, size) or electrical (insulation). It is also possible to synchronize the image captures with the disturbance so as to subtract, by way of the offset image, the same amplitude of disturbance as at the moment of useful image capture. This synchronization is possible only in the presence of a single electromagnetic disturbance or optionally in the presence of several electromagnetic disturbances whose frequencies are multiples of one another. Furthermore, it is necessary to characterize the disturbance and to precisely sequence the control of the photosensitive device as a function of this disturbance, thereby imposing numerous constraints on the design of the photosensitive device. A third solution consists in correcting the image obtained by filtering, for example by means of image correction software. Filtering may, however, lead to the loss or modification of medical information. Moreover, it is difficult to adapt it to a wide spectrum of frequencies of electromagnetic disturbances.